An interaction between mesenchyme and epithelium is required for the normal differentiation of many tissues. The extracellular matrix may mediate this interaction. We propose to investigate several aspects of the relationship between lung alveolar epithelial cells and the underlying mesenchyme and extracellular matrix during fetal lung development. These studies will be performed using rabbit lung obtained from fetuses of varying gestational ages, and an in vitro model, day 19 gestational age rabbit fetal lung tissue explants maintained in culture, a model in which type II cell differentiation is predominant. Ultrastructural morphometry will be used to describe changes in the structure of the basal lamina and physical interactions between mesenchyme and epithelium during fetal lung development. The fetal lung extracellular matrix will be described at the various stages of development by use of light and electron microscopic immunohistochemical staining for 2 components of the extracellular matrix, laminin and fibronectin. In correlative biochemical studies, we will determine the rate of synthesis and accumulation of laminin and fibronectin at different stages of fetal lung development. Once we have determined the normal pattern of changes in the extracellular matrix during fetal lung development, we will examine the effect of cortisol, a hormone which is known to accelerate fetal lung development, on the morphological and biochemical parameters described above. In this manner we can determine if cortisol stimulation of fetal lung maturation is mediated, in part, by changes in the mesenchyme and/or extracellular matrix. In addition, dispersed fetal lung cells will be cultured on artificial extracellular matrices produced by lung endothelial cells or fibroblasts as well as on matrices of laminin, fibronectin and/or type IV collagen. In this way we will determine if extracellular matrix components which appear to play an important role during development in vivo are capable of promoting differentiation in a controlled in vitro environment. The results of these studies will provide important information with respect to interactions between extracellular matrix and epithelial cells during differentiation of fetal lung alveolar epithelium and will be relevant to understanding the mechanisms of tissue differentiation in general. A better understanding of the process of normal fetal lung maturation could allow the development of better methods to stimulate the lung of the right risk-fetus in a manner that could serve to prevent RDS after birth.